Microsystems for biological analyses, their use for detecting analytes, and method for producing them

ABSTRACT

An apparatus for detecting an analyte in a sample, including a cell with at least one fixed electrode, at least one mobile electrode opposite the fixed electrode, the mobile electrode being configured to move with respect to the fixed electrode, and a sample receiving cavity defined by a space between the fixed electrode and the mobile electrode, wherein a surface of at least one of the fixed electrode and mobile electrode facing the sample receiving cavity is configured to bound a ligand of the analyte to be detected. The apparatus also includes a displacement mechanism configured to move the mobile electrode; and an external circuit connected to the fixed electrode and to the mobile electrode, and configured to measure a parameter having a value depending on the presence between the fixed electrode and the mobile electrode of the analyte to be detected.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to microsystems for biological analysesfor use in the health sector, the food processing industry and theenvironmental sector.

It is particularly used to produce biological analyses microsystemsintended for in vitro diagnostics in analyses of infectious illnessessuch as detecting the HIV virus, mycobacteria etc.

In these fields, where it is important to cut costs in the healthsystem, analysis microsystems are currently needed that have a singleusage, that are easy to operate, that use only very small amounts ofsamples and that are based on principles of direct detection thatrequire neither a detection reagent (marker or other means) noramplification of a detection signal.

2. Discussion of the Background

Approximately four years ago new types of tests appeared such asmulti-affinity tests, tests integrating functions such as geneamplification and separation using electrophoresis. The industrialapplications of these new types of tests seem to be widespread eventhough at present they are mainly used in human genome sequencing.

The success of these new tests is mainly due to the micro-technologiesthat have been introduced into the biological field. Through beingintegrated and combined, the micro-technologies have enabled highperformance levels to be reached and speeds and sensitivity levels to beincreased. Furthermore, micro-technologies have led to new technicalsolutions such as miniaturization and integration and new economicsolutions such as mass production, thus giving the development ofbiosensors new impetus.

For example, systems capable of directly performing immunochemical testshave recently been launched. These test systems consist of thin layersof a semiconductor material and silica with antibodies covalently bondedto these layers, said antibodies enabling the presence of an antigen tobe detected that is capable of reacting with the antibodies by measuringthe capacitance of the assembly (see reference 1: Battaillard et al., in"Analytical Chemistry", 60, 1988, pages 2,374 to 2,379). Anothermicrosystem of the same type is described by Schyberg et al. inreference 2: "Sensors and Actuators", B26-27, 1995, pages 457 to 460.

The documents of reference 3: FR-A-598 227 and 4: EP-A-244 326 alsodescribe a method for detecting and/or identifying a biologicalsubstance in a liquid sample by means of electrical measurements.According to this method, the sample is brought into contact with areagent support plate that comprises a specific ligand of the biologicalsubstance to be detected. Said plate can be made of a semiconductormaterial such as silicon and can be coated with an insulating layer ofsilica. The constituents C and/or R are then measured for the electricalimpedance of the system in order to detect whether the biologicalsubstance is present in the sample.

All these systems use the recognition reaction between the biologicalsubstance to be detected and a specific ligand of the substance to beable to detect said biological substance directly without other reagentsor means of detection such as markers, signal amplification reactions,etc. having to be used. This recognition reaction causes an active layerto form that has electrical characteristics, for example a capacitanceand an impedance that are different from those in the system without thesaid layer.

However, the systems that exist at present do not allow for any othermeasurement other than electrical impedance to be used and they are notsuitable for measuring near to or within the active layer.

SUMMARY OF THE INVENTION

The present invention therefore relates to a microsystem for biologicalanalyses that is based on the same principle, i.e. on the recognitionreaction between the biological substance or analyte to be detected anda specific ligand that is capable of detecting this recognition reactionwith several combined types of measurements. Said measurements consistin measuring impedance and rheological viscosity as well as surfaceforces. The microsystem is also capable of measuring near to or withinthe active analyte--ligand layer.

According to the invention the analyte detection or dosage apparatuscomprises:

a cell comprising at least one fixed electrode and at least one mobileelectrode opposite the fixed electrode, the mobile electrode beingcapable of displacement such that it can be brought close to and/or moveaway from the fixed electrode, the space between the electrodesconstituting a receiving cavity for the sample and the surface of atleast one of the electrodes opposite the receiving cavity being capableof bonding a specific ligand of the analyte to be detected,

means for displacing the mobile electrode,

means for connecting the electrodes separately to an external electriccircuit, and

means for measuring the impedance or the electrical capacitance betweenthe electrodes.

Preferably, according to the invention, the mobile electrode and thefixed electrode are both capable of bonding a specific ligand of theanalyte to be detected, the ligands of the two electrodes beingidentical or different.

Generally, at least one and preferably both of the electrodes are coatedwith a ligand.

In the apparatus of the invention, the means for displacing the mobileelectrode can be constituted by means of polarization of the fixedelectrode and the mobile electrode.

By applying a suitable voltage to said electrodes, a capacitance Fc canbe created perpendicular to the surface of the electrodes. Thiscapacitance can be expressed by the following equation:

    Fc=1/2εSV.sup.2 /e.sup.2,

where ε is the dielectric constant of the medium between the electrodes,V is the voltage applied, e is the mean distance between the electrodesand S is the surface area of the electrodes.

This capacitance thus enables the mobile electrode to be displaced,providing that said mobile electrode is free to move within theapparatus.

According to one alternative of the invention the displacement means ofthe electrode are magnetic means.

According to the invention the displacement means of the mobileelectrode can also consist of excitation means using an inductiveeffect. If excitation means are to be used, the mobile electrode can beconnected to coils through which an electrical current passes and saidcoils subjected to a magnetic field such that a force is created thatdisplaces the mobile electrode in the preferred direction.

In one embodiment of the invention the cell comprises a container on asurface onto which the fixed electrode is fastened. The mobile electrodeis positioned opposite the fixed electrode on a movable part mounted onthe surface(s) of the container using at least one flexible beam, suchthat the mobile electrode can be brought close to or move away from thefixed electrode by the beam becoming distorted. The electrical contactsare provided on the surface(s) of the container, on said flexible beamand on the movable part connecting the fixed electrode and the mobileelectrode separately to the external electrical circuit. Said electricalcircuit enables the electrodes to be polarized and the capacitancebetween the fixed electrode and the mobile electrode to be measured.

In this event the displacement means of the mobile electrode can beconstituted by the polarization means of the fixed electrode and of themobile electrode. However, it can be advantageous to use a second pairof electrodes as displacement means of the mobile electrode. Said secondpair of electrodes are positioned respectively on the surface of thecontainer and on the movable part such that they lie opposite oneanother and the polarization means of the pair of electrodes are suchthat they cause the movable part that supports the mobile electrode tomove.

In this particular embodiment of the invention magnetic displacementmeans can also be used to displace the mobile electrode. In this event,said magnetic means can comprise a permanent magnet positioned on themovable part and means for applying a magnetic field to said magnet.

According to this particular embodiment of the invention, the apparatusis advantageously constructed using a silicon substrate that includes anembedded silica layer. In this event, the bottom and the surface(s) ofthe container are made of silicon, the surface(s) are separated from thebottom by a layer of insulating silica, the movable part and theflexible beams are also made of silicon and the electrodes are made ofsilicon that acts as a conductor due to the implantation of ions.

The apparatus of the invention is used to detect or to dose differentkinds of analytes. As an example of the kinds of said analytes,particularly those found in the field of medicine, the invention canapply to antigens, haptens, antibodies, peptides, fragments of nucleicacid (DNA or RNA), enzymes and enzyme substrates.

According to the invention, in order to detect these analytes at leastone of the electrodes is coated with a specific ligand of the analyte tobe detected, for example using direct or indirect grafting of saidligand onto the electrode(s). When the electrode(s) thus coated comesinto contact with a sample containing the appropriate analyte, ananalyte--ligand complex or active layer is created on the electrode(s)and the presence of this complex is detected using various measurements,for example electrical impedance measurements or viscosity measurementsor measurements of the contact force in relation to the sensitive layer.

The specific ligands that coat the electrode(s) are those that have atleast one recognition site of the analyte and are capable of connectingto said analyte. The ligand-analyte pair can therefore belong to thefollowing pairs: antigen-antibody, hapten-antibody, hormone-receptor,DNA-DNA_(c), RNA-RNA_(c), enzyme-substrate or any other combination ofmolecules, whether biological or not, that are capable of creatingcomplexes between themselves.

According to the invention, the use of a detection apparatus comprisinga mobile electrode offers a number of advantages.

The apparatus of the invention enables measurements to be made near toor within the active layer in a position close to the electrodes. Thisresults in heightened sensitivity, enabling the distance between theelectrodes to be small, for example from 0.1 to 0.5 μm.

Under these conditions, signal loss due to the presence of a largerquantity of liquid between the electrodes can be avoided. Accuracy istherefore improved.

On the other hand, when the electrodes are further away from each other,for example at a distance between 1 and 10 μm, the grafting operation ofthe ligand onto the electrode(s) can be made while avoiding theformation of clusters, thus enabling a uniform layer of ligands to beobtained that is as dense as possible.

Similarly, if the sample is brought into contact when the electrodes arein the remote position, the fluidics are simplified by allowingagitation of the fluid on the electrodes, resulting in increasedprobability of recognition between the analyte and the bonded ligand.The formation of clusters is also avoided and a dense, uniform activelayer is obtained on the electrode(s).

Finally, the fact of having a mobile electrode also enables the presenceof an analyte to be detected using other measurements, for example ameasurement of the viscosity of the sensitive layer by studying thedamping of movement of the mobile electrode in said layer after a seriesof alternate movements made by the electrode.

The presence of analyte can also be detected by measuring the mechanicalforce on the surface near the sensitive layer, as will be seen below.

The invention also relates to a method for detecting or dosing ananalyte in a liquid sample that comprises the following stages:

a) introducing said sample in a cell that comprises at least one fixedelectrode and at least one mobile electrode positioned opposite thefixed electrode, said mobile electrode being capable of displacementsuch that it is able to be brought close to or move away from the fixedelectrode. At least one of the said electrodes is coated with a specificligand of the analyte to be detected and the sample is positionedbetween the electrodes with the mobile electrode being in a positionremote from the fixed electrode;

b) displacing the mobile electrode at least once in order for it to bebrought close to the fixed electrode or for it to oscillate between aposition close to and a position remote from the fixed electrode; and

c) during said displacement or after said displacement(s), measuring arepresentative parameter of the formation, or absence of a formation, ofa sensitive layer obtained from reaction of the ligand with the analyteto be detected, said formation being between the electrodes.

In a first embodiment of this method the mobile electrode is displacedonce only or several times in stage b) in order for it to be broughtclose to the fixed electrode. In stage c), the electrical impedancebetween the electrodes is then measured after each displacement, theimpedance measured being compared to a reference value that is set whenthe analyte is not present.

In this first embodiment of the method of the invention, severalimpedance measurements can also be made using different distancesbetween the electrodes in order to obtain a reading of the phenomena ofimpedance variation due to the presence of the active layer formed bythe ligand-analyte complex. The impedance of the intermediary layer andthe impedance of the active layer can also be obtained separately.

In a second embodiment of the method of the invention that is speciallyadapted to measuring the viscosity of the sensitive layer in stage b),the mobile electrode is displaced several times in order for it tooscillate between a close position and a remote position then, aftersaid displacements, the capacitance between the electrodes is measuredover time in stage c).

This enables the damping of the movement of the mobile electrode to bemeasured as well as the difference of amplitude due to the frictioncreated between the mobile electrode and the active layer that isdependent on the viscosity and thickness of said layer. The qualitycoefficient Q, that is directly linked to the loss of energy due to thefriction and therefore to the viscosity, can therefore be calculated.

In a third embodiment of the invention that is particularly suitable formeasuring the surface mechanical force in stage b), the mobile electrodeis displaced once only in order for it to be brought close to the fixedelectrode. In stage c) the capacitance between the electrodes and theforce applied to bring the mobile electrode close to the fixed electrodeis measured.

In other words, the bonding force between the electrodes is measured.Said bonding produces a non-linear electromechanical response from thesystem due to the chemical bonds that are created between the twosensitive layers when they come into contact.

This measuring technique is particularly intended to determine analytesthat have two recognition sites for the same specific ligand or for twodifferent ligands or the mobile electrode and the fixed electrode of acombination of two ligands.

In this version, the mobile electrode and the fixed electrode are coatedwith different ligands or the mobile electrode and the fixed electrodeare coated with a combination of two ligands.

Other characteristics and advantages of the invention will become moreapparent from the following description that is given as anon-limitative example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a detection apparatus according to theinvention.

FIG. 2 is a vertical cross-section of the apparatus of the inventionalong line X-X' in FIG. 1.

FIGS. 3 to 5 show the main preparation stages of the apparatus of FIG.1.

FIG. 6 is a schematic drawing of the apparatus of FIG. 1 during thestage in which the apparatus is coated with electrodes and is broughtinto contact with the sample, the electrodes being in a remote position.

FIG. 7 shows the apparatus of FIG. 1 with the electrodes in a closeposition.

FIG. 8 is a diagram that shows the viscosity of the liquid between theelectrodes depending on the distance between the electrodes, with theanalyte both absent (curve 1) and present (curve 2).

FIG. 9 shows the apparatus of FIG. 1 and the formation of an activelayer with an analyte comprising two recognition sites.

FIG. 10 is a diagram that shows the contact force between the electrodesdepending on the distance between the electrodes, with the analyte bothabsent (curve 3) and present (curve 4).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a perspective view of a microsystem for biological analysesaccording to the invention.

This microsystem comprises a rectangular box-shaped cell 1 that includesa fixed electrode 3 on one of its lateral surfaces 2. A mobile electrode5 is located opposite fixed electrode 3 on a mobile part 7 that ismounted on opposite lateral surfaces 9 and 11 of the cell using thinflexible beams 13 and 15 that constitute a flexible mechanical linkbetween mobile part 7 and surfaces 9 and 11.

The fixed electrode 3 and mobile electrode 5 are electrically isolatedfrom each another by an electrical insulating layer 17 positioned onlateral surfaces 2, 9, 11 and 22 of the cell above cell bottom 19.

The inside of the cell constitutes a receiving cavity for the liquidsample to be analysed.

Electrical contacts 21 and 23 connect fixed electrode 3 and mobileelectrode 5 respectively to an external circuit.

Said external circuit is used to apply suitable voltages to be appliedto the fixed electrode and the mobile electrode, to control thedisplacement of mobile part 7 and the electrode 5 to which it is fitted,to control the distance between the electrodes, and to take variousmeasurements such as the capacitance measurement between the electrodes,possibly against time and during the displacement of the mobileelectrode.

FIG. 2 shows a cross-section of the microsystem of the invention at XX'in FIG. 1.

In this figure, where the parts have the same reference numbers, mobilepart 7 fitted to mobile electrode 5 is shown in the close position tofixed electrode 3.

In FIGS. 1 and 2 mobile part 7 and electrode 5 to which it is fitted areintended to be displaced when suitable voltages are applied toelectrodes 3 and 5. If magnetic means are preferred in order to achievedisplacement, the broken lines shown on the apparatus of FIGS. 1 and 2are modified.

A permanent magnet 8 is positioned on mobile part 7 (shown by a mixeddash/dotted line in FIGS. 1 and 2) and a magnetic field B is createdaround the apparatus, for example using an electromagnet 10 (shown as adotted line in FIG. 2). In this way magnet 8 and mobile part 7 to whichit is fitted are displaced as a result of magnetic field B.

The microsystem of the invention can be achieved using micromachiningprocedures such as those used in microelectronics. Extremely smallapparatuses can thus be produced, for example 3 mm×3 mm on the sideswith a space between the electrodes that can vary between 0.1 μm in theclose position to 10 μm in the remote position.

In FIGS. 3 to 5 a microsystem of this kind is schematically shown thatis based on SOI (Silicon On Insulator) type substrate.

FIG. 3 shows the first stage of the formation of electrodes 3 and 5 inthe SOI substrate that comprises an intermediary layer silica 17 that isapproximately 4000 Å thick and that lies between the lower siliconsection 19 and the upper section 20, also made of silicon and that isapproximately 5 mm thick. To form electrodes 3 and 5, the silicon ismodified locally in upper section 20 by implanting ions, for exampleboron ions, through a mask that constitutes the zones to be implantedand that correspond to the electrodes. In order to achieve theimplantation, sufficient power is used to render the silicon a conductorin the zones up to the intermediary silica layer 17.

External electrical contacts 21 and 23 are achieved using standardtechniques such as those generally used in microelectronics, for exampleusing metallic gold deposition or continuous etching.

The lateral surfaces 2, 9, 11 and 22, flexible beams 13 and 15 and themobile part 7 of the apparatus are then achieved using photolithography,as shown in FIG. 4.

This can be achieved using reagent ionic etching (dry etching) of thesilicon, for example by means of SF₆, down to the layer of silica 17 inthe required motif using, for example, a resin mask according tostandard techniques. The following components are thus obtained: lateralsurface 22, mobile part 7 comprising electrode 5 and lateral surface 2that is fitted with the fixed electrode 3, lateral surfaces 9 and 11 andthin flexible beams 13 and 15 (not shown in FIG. 4).

Preferably, holes 8 are also made in part 7, using photolithography,that will subsequently be used to free mobile part 7 from bottom 19.

In FIG. 5 the stage of the method is shown in which mobile part 7 isfreed. This can be achieved by dissolving the SiO₂ layer that coatsbottom 19 using hydrofluoric acid that enters via the openingspreviously provided between the mobile part 7 and the lateral surfaces 2and 22 and also via holes 8. Layer 17 is thus eliminated except onsurfaces 2, 22, 9 and 11 of the apparatus.

The techniques used in the various stages of production of themicrosystem are those used in microelectronics. The stage that consistsin freeing the mobile part (FIG. 5) can be achieved using the methoddescribed in reference 5: 6 EP-A-605 300.

The use of microtechnologies enables the following:

to heighten the sensitivity of the apparatus (accurate control of thegeometry and optical or electrical parameters),

to improve the detection specificity by using several apparatuses of thesame kind on a single silicon plate (redundancy, multiple detection),

to improve the detection reliability by eliminating problems oflocalized pollution or non-specific reactions, and

to obtain reduced production costs by miniaturizing the sensitive partsand using collective hybridization and packaging techniques developedfor microsystems.

Therefore, the apparatuses produced using microtechnology techniquesprovide the final users, particularly decentralized analysislaboratories, the advantage of being extremely practical.

The microsystem shown in FIGS. 1 and 2 can be used for analyses,particularly for biological analyses, once a specific ligand L of theanalyte A to be detected has been bonded onto one or both electrodes.

Ligand L can be bonded using standard techniques, for example byadsorption onto the electrode once it has been very lightly oxidized orby creating a covalent connection between the electrode and the ligandusing a bifunctional coupling reagent that is capable of reacting withboth the electrode and the ligand.

The use of such reagents is well-known. As an example of a reagentsuitable for coupling ligands constituted by antibodies, proteins andpeptides on the silicon, silane derivatives comprising an alkoxysilanegroup and an NH₂ group may be referred to, the groups being separated bya hydrocarbonated chain. These kinds of techniques are described inreferences 1, 3 and 4 mentioned above.

Said bonding can also be achieved using the techniques described inWO-A-94/22889 (reference 6).

Said bonding is preferably effected before use and with electrodes 3 and5 in the remote position.

FIG. 6 is a schematic drawing of electrodes 3 and 5 and a section ofbottom 19 and of mobile part 7 of the apparatus of FIG. 1. Theelectrodes are in the remote position, at a remote distance d_(e), forexample 2 μm, and they are coated with a specific ligand L.

In order to achieve this bonding the reagents and the ligand arepositioned in the container; this results in the ligand also beingbonded to bottom 19 and to mobile part 7, as shown in this figure.

Once the ligand has been bonded, the sample to be analysed is positionedin the apparatus, the electrodes remaining in the remote position. Ifthe sample contains the analyte A, said analyte constitutes a complex LAwith ligand L or an active layer, the thickness of which is less than1000 Å on the surfaces of the electrodes and on the apparatus, as shownin FIG. 6.

The presence or otherwise of said complex is then verified, for exampleby bringing mobile electrode 5 close to fixed electrode 3 in order toobtain a close distance d_(r) between the electrodes, that is forexample 2,000 Å, i.e. 0.2 μm. A significant parameter is then measured,or is measured when the mobile electrode is brought close to the fixedelectrode.

FIG. 7 shows the apparatus in this position where the distance betweenthe electrodes d_(r) corresponds to the close position. Thus, the activelayer (L-A complex) is seen to occupy most of the space between theelectrodes 3 and 5.

In a first embodiment of the invention this layer is detected using animpedance measurement between electrodes 3 and 5 in the close position.

For this measurement suitable voltage is applied to electrodes 3 and 5and the intensity of the current passing through the cell is measured.By comparing this measurement with a measurement carried out in the sameconditions without analyte, it is possible to verify whether thethickness of the sensitive layer has increased and consequently deductwhether or not analyte is present.

In a second embodiment of the invention analyte is detected in thesample by determining the viscosity of the sensitive layer between theelectrodes. In order to achieve this, the mobile part is subject to aseries of movements towards and away from the fixed electrode byapplying suitable voltages to the electrodes. Then this actuatingvoltage is no longer applied to the mobile part and a voltagemeasurement is applied to determine the variations in time in thecapacitance between the electrodes; said capacitance depends on thedistance between the electrodes and the variations in time result fromthe damping of the oscillating movement produced by the mobileelectrode.

The damping depends on the friction and therefore the viscosity of thelayer of molecules between the electrodes, and particularly on thethickness.

FIG. 8 shows the development of the viscosity η of the liquid relativeto the distance d between the electrodes.

Curve 1 of this figure shows the development without an analyte.

Curve 2 shows the development with an analyte.

Both curves show that the viscosity does not vary significantly when thedistance between the electrodes remains above the thickness of thelayers of ligand alone or the layers of ligand-analyte complex.

On the other hand, when the electrode penetrates the layers theviscosity increases. When no analyte is present and the thickness of thelayers is less, said increase is obtained when the distance between theelectrodes equals thickness d₁.

When analyte is present, this increase in viscosity occurs quicker whenthe distance between the electrodes has d₂ >d₁ thickness because thesensitive layer is thicker than when analyte is absent.

This increase in viscosity can be detected after the mobile electrodehas been oscillated between a close position and a remote position bymeasuring the capacitance variation between the electrodes when themovement of the mobile electrode is dampened. Clearly, the closeposition must correspond to a distance between electrodes that is lessthan d₁.

In a third embodiment of the invention the analyte is detected bymeasuring the contact force between the electrodes.

Said third embodiment is particularly used when the analyte comprisestwo recognition sites for a specific ligand or for two differentspecific ligands.

This example is illustrated in FIG. 9 where fixed electrode 3 and mobileelectrode 5 are shown in close position and are coated respectively withligands L and L' (L'=L or L'≠L) and analyte A or A' that comprises twoidentical recognition sites (A) or different recognition sites (A').Analyte A or A' is connected both to the ligand of electrode 3 and tothe ligand of electrode 5. This causes bonding to occur between theelectrodes.

FIG. 10 shows the contact force f between the electrodes relative to thedistance d between the electrodes.

Curve 3 shows this development without an analyte and curve 4 shows thedevelopment with an analyte. Therefore, the contact force may be seen toincrease quickly when the distance between the electrodes reaches thethickness of the sensitive layer, either d₁ without an analyte or d₂ >d₁with an analyte. This also enables the presence of an analyte to bedetermined.

The contact force can be determined by measuring the capacitance betweenthe electrodes and by measuring the voltage required to bring the mobileelectrode to the close position. The value of the capacitance enablesthe distance between the electrodes to be determined and the forceapplied is in relation to the thickness of the sensitive layer.

As an example, an apparatus of this kind was used to detect the presenceof known DNA sequences in an aqueous solute and using the complimentaryDNA sequence as a ligand. The ligand was bonded by being grafted onto apolymer that had, itself, been electrodeposited onto the fixed electrodeand the mobile electrode.

References:

(1) Battaillard et al., "Analytical Chemistry", 60, 1988, pages 2,374 to2,379.

(2) Schyber et al., Sensors and Actuators, B26 and 27, 1995, pages 457to 460.

(3) FR-A-2 598 227

(4) EP-A-244 326

(5) EP-A-605 300

(6) WO-A-94/22889

What is claimed is:
 1. An apparatus for detecting an analyte in asample, comprising:a cell comprising:at least one fixed electrode, atleast one mobile electrode opposite the fixed electrode, the mobileelectrode being configured to move with respect to the fixed electrode,and a sample receiving cavity defined by a space between said fixedelectrode and said mobile electrode, wherein a surface of at least oneof the fixed electrode and mobile electrode facing the sample receivingcavity is coated with a ligand of the analyte to be detected; adisplacement mechanism configured to move the mobile electrode; and anexternal circuit connected to said fixed electrode and to said mobileelectrode, and configured to measure a parameter having a valuedepending on the presence between said fixed electrode and said mobileelectrode of the analyte to be detected.
 2. The apparatus of claim 1,wherein said fixed electrode and said mobile electrode are coated with aligand of the analyte to be detected.
 3. The apparatus of claim 1,wherein the displacement mechanism comprises a polarization mechanismconfigured to polarize the fixed electrode and the mobile electrode. 4.The apparatus of claim 1, whereinthe cell comprises a container, saidfixed electrode being fastened to one surface of said container, thedisplacement mechanism comprises a mobile part mounted on surfaces ofthe container by at least one flexible beam, and said mobile electrodeand said fixed electrode are connected to electrical contacts providedon surfaces of the mobile part and flexible beam.
 5. The apparatus ofclaim 4, wherein the displacement mechanism comprises a polarizationmechanism configured to polarize the fixed electrode and the mobileelectrode.
 6. The apparatus of claim 4, wherein:the displacementmechanism comprises a third electrode and a fourth electrode provided ona surface of the container and on the mobile part respectively such thatsaid third and fourth electrodes are positioned opposite each other, andsaid displacement mechanism is configured to polarize said third andfourth electrodes so as to move the mobile electrode.
 7. The apparatusof claim 4, wherein the displacement mechanism comprises a magneticdevice.
 8. The apparatus of claim 7, wherein the magnetic devicecomprises a permanent magnet positioned on the mobile part and amagnetic field generator configured to apply a magnetic field to saidmagnet.
 9. The apparatus of claim 4, wherein:the container has a bottomand lateral surfaces comprising silicon, the lateral surfaces areseparated from the bottom by a layer of insulating silica, the mobilepart and the flexible beams comprise silicon, and the electrodescomprise silicon with implanted ions.
 10. A method for detecting ananalyte in a liquid sample, comprising the steps of:a) introducing saidsample in a cell that comprises at least one fixed electrode and atleast one mobile electrode positioned opposite the fixed electrode, saidmobile electrode being configured to move with respect to the fixedelectrode, at least one of the fixed electrode and the mobile electrodebeing coated with a ligand of the analyte to be detected, and the liquidsample being positioned between the fixed electrode and the mobileelectrode with the mobile electrode being in a remote position from thefixed electrode; b) displacing the mobile electrode at least once fromthe remote position to a close position; and c) measuring a parameterhaving a value depending on the presence of a layer formed from areaction of the ligand with the analyte to be detected, said layer beingbetween the fixed electrode and the mobile electrode.
 11. The method ofclaim 10, further comprising coating the fixed electrode and the mobileelectrode with a ligand.
 12. The method of claim 10, wherein measuringsaid parameter comprises measuring the electrical impedance between thefixed electrode and the mobile electrode after each displacement, andfurther comprisingd) comparing the measured electrical impedance to areference determined in the absence of an analyte.
 13. The method ofclaim 10, wherein:displacing the mobile electrode comprises displacingthe mobile electrode several times in order for said mobile electrode tooscillate between the close position and the remote position, andmeasuring said parameter comprises measuring the time capacitancebetween the fixed electrode and the mobile electrode.
 14. The method ofclaim 10, wherein measuring said parameter comprises measuring thecapacitance between the fixed electrode and the mobile electrode duringdisplacing the mobile electrode, and further comprises measuring theforce applied to bring the mobile electrode closer to the fixedelectrode.
 15. The method of claim 14, further comprising:covering thefixed electrode with a first ligant configured to bind to a firstrecognition site of the analyte to be detected, and covering the mobileelectrode with a second ligant configured to bind to a secondrecognition site of the analvte to be detected.
 16. The method of claim14, further comprising coating the fixed electrode and the mobileelectrode with two combined ligands, each configured to bind to arecognition site of the analyte to be detected.
 17. The method of claim10, wherein displacing the mobile electrode is performed so that amaximum distance between the fixed electrode and the mobile electrode isfrom 1 to 10 μm and a minimum distance between the fixed electrode andthe mobile electrode is from 0.1 to 0.5 μm.
 18. The method of claim 10,further comprising seleting an analyte-ligands pair from a groupconsisting of antigen-antibody, hapten-antibody, hormone-receptor,DNA_(c) -DNA, RNA_(c) -RNA and enzyme-substrate, said layer between thefixed electrode and the mobile electrode comprising said analyte-ligandspair.
 19. A method of manufacturing the apparatus of claim 9, comprisingthe steps of:1) forming electrical conducting zones on a siliconsubstrate comprising an embedded layer of silica, said electricalconducting corresponding to the fixed and mobile electrodes and beingcreated by the implantation of ions through a mask; 2) etching thesubstrate down to the embedded silica layer in order to form, surfacesof the container, the mobile part and the flexible beams; 3) eliminatingthe embedded silica layer except for sections that constitute thesurfaces of the container; 4) forming electrical contacts on thesurfaces of the container and the flexible beam that connect the fixedand mobile electrodes separately to the external circuit; and 5)bounding a ligand on a surface of at least one of the fixed electrodeand mobile electrode.